Photographic medium and methods of preparing same



Nov. 23, 1965 5 u 'VALLE L 3,219,448

PHOTOGRAPHIC MEDIUM AND METHODS OF PREPARING SAME Filed 001:. 23. 1962 FIG.

FIG.3

FIG.4

FIG.2

FIG.6

INVENTOR JAMES E. LU VALLE GERSHON M. GOLDBERG JOHN 6. PACK Y Roam. M3.

ATTORNEYS United States Patent Office 3,219,448 Patented Nov. 23, 1965 3,219,448 PHOTOGRAPHDC ll iEDlUM AND METHODS F lREPARlNG SAME James E. Lu Valle, Stony Brook, N.Y., and Gershon M. Goldberg, Arlington, and John G. Pack, Reading, Ivlass, assignors, by mesne assignments, to Technical Operations, Incorporated, a corporation of Delaware Filed Oct. 23, 1962, Ser. No. 233,197 21 Claims. (Cl. 9661) This invention relates in general to the field of photographic materials, and more particularly to silver halide photographic materials. Specifically, the present invention is directed to photographically responsive ultra-thin layers of silver halide, such as are prepared by evaporation of the silver halide from a molten pool, and the condensation of these vapors on an appropriate substrate material. The resultant ultra-thin stratum of condensed silver halide is formed of a large number of micro-crystals which are supported on the substrate primarily by being adhered directly to each other and directly to the substrate. Such material is therefore binder-free as distinguished from conventional gelatin type photographic materials; and even if a retaining surface layer is applied over the binder-free material, the silver halide is still substantially binder-free.

Photographic media which are now known and in use are generally characterized by an emulsion or gelatin in which aggregates of photosensitive material are suspended. The use of an emulsion to hold photosensitive material on a supporting surface has many disadvantages. Among these is the fact that developing agents must penetrate the emulsion to reach the photosensitive material. Another disadvantage is the fact that there is a limit to the minimum grain size that can be achieved, due in turn to the fact that the aggregates of photosensitive material which are suspended in the emulsion cannot individually be reduced beneath a certain size without losing or suifering diminution of their photographic properties. Still another is the fact that a substantial portion of the area of the photographic medium consists of gelatin, rather than photosensitive material, and this fact coupled with the minimum limit on grain size places a limitation on the fineness of detail that can be recorded. Further, the aggregates themselves are in the nature of particles of photosensitive material entrapped in globules of gelatin, so that each aggregate has the chemical characteristics of the emulsion as well as its photographic characteristics. Among the characteristics of gelatin emulsions are sensitivity to radioactive energy, such as gamma rays, and a tendency to pick up moisture, both of which cause fogging of negatives and shorten the storage life of photographic media. Further, as is well known, a gelatin, once wet, cannot be quickly dried without taking special measures which are costly and tend generally to harm the photographic image, and, as is also well known, special measures are required to bond a gelatin, which is hydrophilic in nature, to a film base, which is hydrophobic in nature. Another distinct disadvantage of known emulsion-type films is the fact that they can be used only once.

It is an object of the present invention to provide a photographic medium which does not require the use of an emulsion surrounding the photosensitive grains, which affords a finer grain structure than has heretofore been available, which is, by comparison with prior media, essentially grainless, and which is substantially insensitive to nuclear radiation. It is another object of the invention to provide a photographic medium which lends itself to image-transfer processes and can be re-used a number of times. A further object is to provide a method of manufacture of photographic media which does not require the use of emulsion or other vehicles for preparation of, or

deposition of photosensitive material on supporting surfaces, which is capable of precise control and which is economically competitive with or superior to existing methods of preparing conventional emulsion-supported photographic media. It is a further object of the invention to provide a non-gelatinous or non-emulsion, or binder-free type silver halide photographic medium and a process of preparing the same.

According to the invention, a photographic medium, chosen from the group of silver chloride, silver bromide and silver iodide, and combinations of these, is evaporated in near vacuum and deposited by condensation in a microcrystalline form directly on a surface of a supporting medium, under controlled conditions of temperature, pressure and time. The supporting medium may be the surface of a solid material, such as glass, photographic quality paper, or a photographic quality plastic (e.g., polyester, cellulose derivative, polymer or super polymer) film. Alternatively, the supporting medium may be coated on the silver halide receiving surface with a bonding agent, such as a gelatin, a lacquer, or a normally tacky adhesive, in order to cause the evaporated silver halide to adhere more securely thereto. As another alternative, the surface of a solid material such as glass (or other vitreous or siliceous material) may be first treated with a solid bonding agent, for example, silicon monoxide, to effect better adhesion between the substrate and evaporated silver halide when the latter is deposited thereon. Preferably, the evaporation temperature should be below the decomposition temperature of the silver halide starting material and above its melting point; and the condensation temperature should be above room temperature, preferably between about 30 C. and about 50 C. The pressure of the evaporation and condensation process can be varied over wide limits; however, a pressure in the range of approximately 10- to 10 millimeters of mercury is preferred. The thickness of the resultant layer or stratum of microcrystalline silver halide photographic material which can be used can likewise vary, but should be a very thin film. Some degree of photographic utility can be obtained with evaporated silver halide strata as thick as about 3.5 microns; on the other hand, it has been found that generally there is a substantial and rapid fall oil of photographic properties with thicknesses in excess of an amount around 0.3 micron. It has been found that at silver halide stratum thicknesses of around 0.3 micron one usually obtains maximum photographic properties, i.e., maximum density, speed, and gamma. Depending upon the exact conditions of temperature and pressure during the evaporation and condensation of the silver halide, and the particular substrate employed to collect the silver halide vapors, the exact thickness at which maximum photographic properties is obtained will vary about 0.1 or 0.2 micron, or thereabouts, to either side of the stated 0.3 micron thickness. However, the fall olf of photographic response is usually quite rapid to either side of that thickness which provides the maximum response. Accordingly, the preferred range of thickness for the evaporated silver halide stratum is from around 0.3 micron to around 3.5 microns, and it is considered that the optimum range for most photographic purposes is in the lower end of this range of a few tenths of a micron, particularly around the 0.3 micron value.

Thus, in accordance with the present invention, there is provided a photographic medium consisting essentially of a continuous layer of microcrystalline photographic material, which is relatively grainless, or may be termed superfine grain, as compared with prior photographic materials prepared in suspension in an emulsion. This new photographic medium has the advantages, heretofore not available, that it can be developed more easily by liquid and even gaseous developers, since there is no necessity for the developer to penetrate a gelatinous matrix to reach the photographic medium itself, that it lends itself to processes of image transfer and re-use after such transfer, and that it is practically insensitive to ionizing nuclear radiation.

Photographic media according to the invention may be made of a single one of the above-mentioned halides, or of two (or more) of them. In the latter case starting quantities of each desired halide are vacuum evaporated, in the same location or separate locations, their vapors mixed in the low. pressure region if they started in separate locations, and their mixed vapors are condensed on a surface on the supporting medium. Thus, starting with appropriate quantities of silver chloride and silver bromide, a layer of silver chlorobromide is deposited. Or, starting with appropriate quantities of silver bromide and silver iodide, a layer of silver bromoiodide is deposited. Like the single halide layer, these layers are of substantially homogeneous microcrystalline form.

Other and further objects and features of the invention will become apparent from the following description of certain embodiments thereof and of methods and apparatus for preparing them. This description refers, for purposes of illustration, to the accompanying drawings, wherein:

FIG. 1 is a top view of a portion of a known vacuum evaporation machine which is useful in practicing the method of the invention;

FIG. 2 is a schematic side view of a portion of such a machine;

FIGS. 3, 4, and 5 are, respectively, enlarged sectional views of photographic media made according to the invention;

FIG. 6 is a sectional view on a still larger scale of photographic film grains made according to known emulsion processes; and

FIG. 7 is a similar sectional view of a polycrystalline photographic medium made according to the present invention.

FIG. lshows the table 11 of an existing machine for the vacuum deposition of metals and similar materials. A basic machine of the kind referred to is illustrated and described in the book Vacuum Deposition of Thin Films, by L. Holland, published by John Wiley and Sons, Inc., New York City, 1948, pages 7 and 8. A vacuum coating machine model LC1-14A of the Consolidated Electroydnamics Corporation was used in achieving some of the results mentioned below. This machine has a belljar 14 (FIG. 2) about 13 inches in diameter and 24 inches in height on the table 11, under which a low-pressure, near-vacuum, region is provided. The location of the bell-jar when in place on the table 11 is indicated by the dashed circle 12. The space under the bell-jar is exhausted through an opening 13 in the table 11.

Electric power terminals 15, 16, 17 and 18, 19, 20 (FIG, 1) are provided on the table 11 for supplying current for melting the silver halide source material to be evaporated in vacuum, and a pair of auxiliary terminals 21and 22 (FIG. 1) supply operating voltage (e.g., 110 volts, A.C.) for auxiliary devices. These terminals are all within the locus 12 of the rim of the. bell jar. A first electrically conductive container, boat, or filament, 24, which may be made of molybdenum, tantalum or tungsten, for example, is connected by two stiff electrical conductors 25 and 26 to a pair of the power terminals 15 and 18, respectively. The conductors 25 and 26 also support the open boat or filament 24 in a fixed position above the table 11. Bolts 15.1 and 18.1 (FIG. 2) fasten the free ends of these conductors to the two power terminals 15 and 18, which are usually threaded for that purpose. The starting material (not shown), to be vacuum evaporated is placed in the open boat or filament 24.

When, in practicing the invention, it is desired to vacuum evaporate two quantities of starting material in separatelocations, a second boat or filament 31 can be employed, supported on two conductors 32 and 33, as is shown in FIG. 1. These conductors may be connected to two separate power terminals 16 and 19, respectively, as shown in FIG. 1, or if desired they may be connected to the same power terminals 15 and 18 as the first filament 24. With two filaments connected to separate pairs of terminals as in FIG. 1, it is possible to control the current to each filament independently. Obviously, a third filament (not shown) can be added, connected to a third pair of power terminals 17 and 20, if desired.

In carrying out the method of the invention, a body or substrate 45 (FIG. 2), to be coated with the condensate from the vapor of a material (not shown) vacuum evaporated from the filament or boat 24, is supported by any suitable means (not shown) within the bell jar 14 above the filament 24 at a suitable distance therefrom. The starting material is at least one silver halide, chosen from the group of silver chloride, silver bromide and silver iodide. If only one of these compounds is to be coated on a surface on the body 45 a single filament 24 will suflice. If two of these compounds are to be coated simulataneously on the body 45, for example, silver chloride and silver bromide, or silver bromide and silver iodide, a quantity of each compound may be placed on the single filament 24, or two filaments 24 and 31, as in FIG. 1, may be employed and a quantity of each compound may be placed separately on each filament. The region under the bell jar 14 is evacuated to a low pressure, preferably in the range of about 10* to about l0 millimeters of mercury, although pressures within wider limits, from 0.1 millimeter of mercury to less than 10* millimeters of mercury can be used. It is preferred to evacuate the region under the bell jar to the working pressure prior to applying heating current to the filament 24 (and filament 31, if used), so that the desired conditions of temperature are not prolonged and can be controlled to a value at which evaporation takes place without any significant decomposition of the starting materials. This is particularly true with respect to silver iodide, which melts at a temperature above 500 C., and decomposes at or near its melting point. On the other hand, silver bromide melts at approximately 434 C. and decomposes at or near its melting point. On the other hand, silver bromide melts at approximately 434 C. and decomposes at 700 C., while silver chloride melts at 455 C. and decomposes at about 1550 C. so that close control of the evaporation temperature is not as critical with these compounds as it is with silver iodide. The body 45, which is the target for the vapors of the starting material, is spaced from the filament 24 (and filament 31, if used) a distance such that condensation of the vapors occurs at a condensation temperature above room temperature, preferably in the range of 30 C. to 50 C. At a pressure in the range of 10- to 10 millimeters of mercury, the temperature in the region above the filament 24 is substantially in this range at a distance about 3 /2 inches from the filament, as measured by a copper-constantan thermocouple. Under these conditions, the process is carried on for a period of time, about one minute or less to one-half hour, depending upon the temperature and pressure conditions selected, until the starting material has been deposited on the target body 45 to a desired thickness.

The substrate body 45 as shown in FIG. 2, may be a sheet of glass 45.1 as shown in FIG. 3. Alternatively, it may be a sheet of paper, plastic film, or other conventional and suitable photographic quality substrate material. The silver halide starting material evaporated from the filament 24 in FIG. 2 condenses on a surface 47 of the substrate sheet as a micro-crystalline coating or layer 46. As the silver halide vapors condense on this surface 47, small crystal particles form and coalesce to form a tightlypacked layer wherein the crystals are supported on the substrate by being adhered directly to each other and to the substrate without the need of a binder. The density of a layer formed in this manner, has been measured as follows:

Using a body 45 masked by a shield having an aperture which was a square 5.7 cm. on each side, a layer of silver bromide was deposited on a surface on the body through the aperture to a mean thickness of 2.2 microns as measured by a spectrophotometer using the method of optical path differences between reflections of controlled light from the front and back surfaces of the layer. In this example, the thickness of the polycrystalline layer of silver bromide varied from 1.94 microns at the edge to 2.24 microns at the center. The body 45 was weighed before and after coating to obtain the weight of the layer. The volume of the layer was calculated from the dimensions of the aperture and the mean thickness, and found to be:

The weight of the silver bromide layer was found to be 0.44 gram. From these values, the density of the silver bromide layer was calculated to be 6.16 grams/cc. The density of solid silver bromide crystals is 6.47 grams/ cc. as given in the Handbook of Chemistry and Physics, 14th edition, page 265. The ratio of the densities of this polycrystalline layer to solid crystals is therefore percent. This indicates that the layer 46 is very tightly packed and has a density closely comparable to that of the solid crystal of the starting material.

The polycrystalline layer of silver halide 46 can be deposited also on a bonding type of substrate, as suggested above, one form of which is shown in FIG. 4. In this figure, the supporting body 45.2 is illustrated as made of synthetic resin or plastic, for example, a film made of a cellulose derivative, a polymer, a super polymer or a polyester. This film has on one surface a subbing stratum 48 of soft material, such as a layer of shellac or a water-permeable colloid, for example, gelatin. The evaporated layer of silver halide is deposited on the stratum 48. Due to the softness of the stratum 48, the first crystal particles of the silver halide which are deposited on it penetrate somewhat, as is illustrated at 46.1, and the coalescence of subsequent particles of the silver halide on the initial particles serves to bind the silver halide crystal layer 46 to the supporting body 45.2 more securely than had the stratum 48 not been employed. Obviously, the glass body 45.1 in FIG. 3 and the synthetic resin body 45.2, of FIG. 4 may be interchanged if desired.

One example of a photographic medium we have made according to FIG. 4 is as follows:

The microcrystalline silver halide layer 46 was silver bromide; the supporting body 45.2 was made of a ce lulose acetate film; and the subbing layer 48 was a coat of a commercially available lacquer known as Carpenter Morton Larcoloid No. 1903 clear glass lacquer, packaged in a pressurized spray container. The contents of the container were as follows, according to its label:

Another example of a photographic medium we have made according to FIG. 4 is as follows:

The microcrystalline silver halide layer 46 was silver bromide; the supporting body 45.2 was made of polyethylene terephthalate with the subbing layer 48 attached to it, a form which is available commercially under the trade mark Cronar from E. I. du Pont de Nemours & Co., Wilmington, Delaware, and in which the subbing layer is believed to be a water-permeable colloid layer as described in U.S. Patent No. 2,627,088 or No. 2,698,242. Cronar in this form is sometimes known as Subbed Cronar. The layer of silver bromide 46 was deposited on the substratum 48, according to the present invention. Cronar film is a light-transparent version of polyethylene terephthalate which is 4 mils (0.004 inch) thick. Subbed Cronar is commercially available for the manufacture of gelatinous photographic media. It is useful, but not required in making photographic media according to the present invention.

A third example of a photographic medium which we have made according to FIG. 4 employing a soft subbing layer 48 is one which was made by vacuum evaporation and vapor deposition of silver bromide onto the sticky side of Scotch Tape polyester film No. 850, a pressure sensitive adhesive tape which is commercially available. The normally tacky adhesive which is part of this tape, of which the compoistion is not known to us, constitutes the soft subbing layer 48.

In FIG. 5 the supporting body 45.1 is made of glass or similar vitreous material, on which a solid subbing layer 49 is coated. The subbing layer may be made, for example, of silicon monoxide, coated on the glass body 45.1 as a thin layer under vacuum evaporation conditions. The microcrystalline layer of silver halide 46 is deposited on this subbing layer, which serves to bind the silver halide layer to the supporting body 45.1 more securely than would be obtained in the absence of such subbing layer.

We have found that photographic media made by vacuum evaporation of silver bromide for example, according to the invention can be exposed to gamma radiation without significant change in the fog level. Two samples of such a photographic material, of equal sensitivity, were compared by exposing one sample to 10,000 R of gamma radiation and then developing both. On development there was no significant change in fog level from the sample which was so exposed to the sample which was not exposed to gamma radiation. While we do not wish to be limited by any particular explanation of this phenomenon, we believe it can be explained as follows, reference being made to FIGS. 6 and 7 of the accompanying drawing:

Conventional photographic media are prepared as a suspension of particles or grains of photographic material in a hydrophilic emulsion, or a water-permeable colloid, such as gelatin. The resulting coating of photosensitive material consists of for example, the gelatin disposed around and between the grains of photographic material, as is illustrated in FIG. 6, which shows diagrammatically a group of closely-packed globules 51 of gelatin in which crystal grains or particles 52 of the photographic medium are suspended essentially without touching each other. On the other hand, photographic coatings according to the invention consists entirely of tightly-packed crystal particles 53 as is shown in FIG. 7. These particles coalesce on each other during the process of condensing from the vapor state. In emulsions the gelatin contained around and within the grains 52 serves as a halogen acceptor and prevents recombination of silver and halogen atoms. In our medium (FIG. 7) the absence of halogen acceptor within the body of the medium allows recombination to take place and, thus, any damage caused by the radiation is self-repairing.

The vacuum evaporation process itself is thought to act at least in part as a purifying process, so that the coating can consist essentially of a chemically pure layer of the starting silver halide, that is, a layer in microcrystalline form in which the individual particles are homogeneous and substantially free of foreign substances. Obviously, however, where a substantially pure silver halide layer i desired, it is nevertheless advantageous to start with purified starting materials. By the term chemically pure we mean that the layer of photographic material prepared according to this invention has less than approximately 100 parts per million of any single other substance. This layer is not affected by gamma radiation, as is indicated above. Thus, the vacuum evaporation method of this invention confers on photographic materials a property of radiation resistance which is not afforded by any prior-known coated photographic material. As is mentioned above, the density of photographic media prepared according to the invention is approximately 95 percent as great as the density of solid crystals of the same material, which is a measure of the closeness of packing or coalescence of the individual particles 53 in FIG. 7.

We have made photographic media according to FIG. 4 of mixed silver halides made from separate quantities of starting silver halides separately vacuum evaporated from two filaments 24 and 31 according to FIG. 1. In one case silver bromide and silver iodide were used, the silver iodide being the minor portion (not more than ten percent) of the total quantity of starting silver halide, the vapors of these two compounds were allowed to mix in the low pressure region between the filaments and the target; the resulting microcrystalline silver halide layer 46 consisted of silver bromoiodide. In another case, silver chloride and silver bromide were used, silver chloride being the major portion of the total quantity of starting silver halide; the vapors of these two, after mixing in the loW pressure region between the filaments and the target, condensed on the target to form a microcrystalline layer 46 of silver chlorobromide.

In all cases described above, the pressure, temperature of condensation and evaporation times were within the preferred ranges set forth above. We have found that a photographic medium according to FIG. 4, made of silver bromide, under control parameters within these ranges, has appreciable sensitivity, of the order of ASA 0.01. It is preferred to reach equilibrium conditions during the evaporation. We have reduced the evaporation time to 15 seconds and achieved a silver bromide photographic medium which yielded good step wedges, without fogging, when exposed in a sensitometer and developed in a standard developer, and the evaporation time can be even further reduced. To obtain the preferred operating parameters with these short evaporation times, a shutter (not shown) may be provided between the filament 24 and the target 45 (FIG. 2) together with remotely controllable means to remove the shutter; for the latter purpose use may be made of the auxiliary terminals 21 and 22 (FIG. 1). Then the filament or boat 24 may be heated and the operating conditions established before any of the silver halide vapor is permitted to reach the target, so that the process of the invention can be carried out, even for a short time interval, under essentially stabilized conditions.

Thermal processing conditions in the Consolidated Electrodynamics vacuum coating machine mentioned above are approximately as follows:

(a) using a tungsten filament or boat 24 (or 31) one mil-inch thick, one-half inch wide and 2% inches long, starting quantities of silver bromide, silver chloride and silver iodide were successfully vaporized with an electric power input about 28 to 32 watts, depending on the mass of the starting material; the desired power level was achieved with a wide variety of the current and voltage settings available with this machine, and at pressure anywhere between 0.1 millimeter of mercury and approximately 10- millimeters of mercury, which latter was the practical low-pressure limit of the particular machine in use; and

(b) measured with copper-constantan thermocouple-s, at a pressure about or 10- millimeters of mercury, the temperatures in the region above the filament men- Temperature, C. Distance above the Filament (Inches) Range (1) Range (2) 1% Inches 74-90 2 8 55-70 72 35-50 55 25-35 37 Range (1) indicates the range of temperatures achieved in about twenty minutes; range (2) indicates the temperature after one-half hour, which appeared to be essentially an equilibrium temperature. The target (45 in FIG. 2) is preferably located at a distance about 3 /2 inches above the filament under these thermal operating conditions.

(c) measured with a chromel-alumel thermocouple, at a pressure about 10- or 10- millimeters of mercury, a six-gram starting sample of silver bromide in the filament mentioned above had a temperature in the melted condition which was in the region from 460490 C. over a six-minute interval (Range (1)), and stabilized at about 515 C. in Range (2); for this measurement the thermocouple was inserted in the pool of melted silver bromide.

The operating pressure can be varied over limits much wider than the preferred region of about 10 to about 10 millimeters of mercury. For example, a pressure of 0.1 millimeter of mercury can be used. However, under the preferred thermal conditions, the processing time at this pressure is about one-half hour. As a practical matter, it is preferred to employ a pressure in the preferred range, which permits the processing time to be substantially reduced. Generally, we use processing times as little as about one-minute or less and as long as about fourteen minutes.

Thus, proceeding in accordance with the foregoing parameters, silver bromide for example is evaporated from the boat or filament at the suggested pressure of between 10* and 10 mm. of Hg, and at the suggested temperature of about 515 C., onto a Cronar substrate. If the evaporation is conducted for about 1 minute as suggested above, the resultant microcrystalline photo graphic layer of silver bromide formed is approximately 0.3 micron thick. Under these conditions, if the silver bromide is evaporated for only 15 seconds as suggested hereinabove, the microcrystalline layer is approximately 0.1 micron thick. If the evaporation is permitted to proceed for about fourteen minutes as suggested above, the silver bromide layer is about 3.5 microns thick.

While, as is indicated above, Subbed Cronar is a suitable material for collecting the microcrystalline layer 46 and retaining it during subsequent processing (i.e., development) stages, other materials can be used, some of which are described above. Unlike gelatinous photographic media, the media according to the present invention do not present the problem of adhering a hydrophilic material to a hydrophobic material, so that a wider range of bonding substrates can be used in carrying out the process of this invention. For some purposes, the bonding substrate is dispensed with entirely.

The photographic media made according to the invention are particularly suitable for use in image-transfer processes. Upon exposure and development, the silver halide microcrystals which are developed are converted essentially to pure silver particles. It the stratum of silver halide is sufficiently thick that the development process does not progress through the entire thickness to the substrate, then the developed silver particles are resting on a substrate of the original silver halide. The adhesion between silver particles and silver halide microcrystals is poor, and by contacting the developed medium with a sheet coated with a normally tacky material the silver particles are easily removed from the unexposed silver halide layer. Upon separation of the contacting sheet from the photographic medium, there results a transfer of the developed image to the contacting sheet, consisting essentially of developed silver particles which do not require fixing, and the microcrystalline layer of silver halide remaining on the original photographic medium can be re-used, after suitable treatment. The original photographic medium can be used in its original state of sensitivity, or it can be treated to impart enhanced sensitivity prior to use. The number of times a given sample of the photographic medium of the invention can be reused in this manner will depend in part upon the thickness of the original layer of silver halide 46.

If it is desired to fix an image on a sample of the original photographic medium, the use of a layer 46 sufiiciently thin to result in development of the image through the entire thickness of the layer will prevent removal of the image when the remaining silver halide is dissolved in the fixing bath. Otherwise, upon dissolution of the underlying silver halide, the silver image floats off. Alternatively, a retaining sheet, similar to a transfer sheet as described above but liquid pervious, can be first applied to retain the developed image, and then dissolution of silver halide in the fixer will not cause loss of the developed silver particles. Rather than employ such expedients as this, it is preferred to use silver halide strata which are suificiently thin to result in development of light struck areas down to the supporting substrate. Since the crystal structure resulting from the present process of evaporating silver halide provides a continuity of contact between grains both laterally across the face of the photographic material as well as downwardly through the silver bromide layer to the substrate support material, the development process spreads laterally from a light struck to adjacent grains as well as downwardly through the thickness of the layer. Therefore, if the thickness of the silver halide layer is too great, development down to the substrate can result in an intolerable loss of acutance and image sharpness. It has been found that a silver halide layer of about 0.3 micron in thickness can be developed down to the substrate without appreciable loss of image sharpness and without fogging.

In view of the fact that maximum speed, density, and gamma, are all obtained with about a 0.3 micron thick layer of silver halide, and since the ability to develop down to the substrate without noticeable loss of image sharpness or fogging is likewise obtained at this thickness of 0.3 micron, it is apparent that this thickness provides the approximate optimum for the photographic material of the present invention. These factors however are not intended to exclude entirely slightly thicker layers of silver halide, of say up to about 3.5 microns, from the scope of this invention, since such layers can be used where the maximum properties are not essential or desired, and where the feature of multiple exposure with image transfer outweighs the loss of photographic properties suffered.

The embodiments of the invention which have been illustrated and described herein are but a few illustrations of the invention. Other embodiments and modifications will occur to those skilled in the art. No attempt has been made to illustrate all possible embodiments of the invention, but rather only to illustrate its principles and the best manner presently known to practice it. Therefore, while certain specific embodiments have been described as illustrative of the invention, such other forms as would occur to one skilled in this art on a reading of the foregoing specification are also within the spirit and scope of the invention, and it is intended that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

What is claimed is:

1. A silver halide photographic element comprising a substrate sheet, a stratum of substantially binder-free, vapor deposited silver halide microcrystals supported upon and substantially covering in substantially continuous phase a surface area of said sheet, said stratum having a thickness of from about 0.1 to about 0.5 micron and a density of less than that of said halide in solid crystalline form,

2. An element as set forth in claim 1, wherein said silver halide is silver bromide.

3. An element as set forth in claim 1, wherein said silver halide comprises a plurality of halides.

4. An element as set forth in claim 1, wherein said density is about of that of said halide in solid crystalline form.

5. An element as set forth in claim 1, and further having a protective layer of material overlying said stratum and cohered to the surface thereof and sandwiching it between said layer and said substrate.

6. An element as set forth in claim 1, wherein said thickness is about 0.3 micron.

7. A method of forming a silver halide photographic element, comprising evaporating silver halide under a high vacuum with said silver halide at a temperature in excess of its melting point, and condensing the silver halide vapors over a surface area of a base member to form a layer of light responsive silver halide upon said base as a support therefor, said evaporation being conducted from a pool of molten silver halide, said evaporation and con densation being effected under substantially stable conditions of pressure and temperature to afford substantially uniform characteristics to the silver halide deposit, and wherein said silver halide is deposited to a thickness of from about 0.1 to about 0.5 micron.

8. A method as set forth in claim 7, wherein the pressure of said vacuum is less than about 0.1 mm. of mercury.

9. A method as set forth in claim 8, wherein said pressure is between about 10- and about 10 mm. of mercury.

10. A method as set forth in claim 7, wherein said temperature is below that at which substantial decomposition occurs 11. A method as set forth in claim 7, wherein the pressure of said vacuum is between about 10- and about 10* mm, of mercury, and said temperature is between about 500 and about 700 C.

12. A method as set forth in claim 7, wherein said thickness is about 0.3 micron.

13. A method of photography comprising exposing to an optical image a layer of substantially binder free silver halide microcrystals supported on a substrate, said layer being from about 0.1 to about 0.5 micron thick, developmg said layer until substantially all of the light struck areas are developed through the thickness of the layer down to the substrate, and fixing the developed image.

14. method as set forth in claim 13, wherein said fixing is effected by dissolving from said layer substantially all the residual silver halide.

15. A method as set forth in claim 13, wherein said layer is about 0.3 micron thick.

16. A silver halide photographic element comprising a stratum of substantially binder-free, vapor deposited silver halide microcrystals substantially covering and supported upon an area of a substrate, said stratum having a density less than that of said halide in solid crystalline form, and a protective layer of material overlying said stratum and cohered to the surface thereof and sandwiching said stratum between said layer and said substrate.

17. An element as set forth in claim 16, wherein said silver halide is silver bromide.

18. An element as set forth in claim 16, wherein said silver halide comprises a mixture of halides.

19. An element as set forth in claim 16, wherein said stratum is about 0.3 micron thick,

20. An element as set forth in claim 16, wherein said density is about 95% of that of said halide in solid crystalline form.

References Cited by the Examiner UNITED STATES PATENTS De Boer et a1 9694 De Beer et a1 9694 De Boer et a1 117106 X Strong 117106 X 12 FOREIGN PATENTS 802,041 9/ 1958 Great Britain.

OTHER REFERENCES Yamada et al.: Chemical Abstracts, vol. 50, pp. 6979- 81 (1956).

Yamada et 211.: Chemical Abstracts, vol. 50, pp. 11157- NORMAN G. TORCHIN, Primary Examiner. 

1. A SILVER HALIDE PHOTOGRAPHIC ELEMENT COMPRISING A SUBSTRATE SHEET, A STRATUM OF SUBSTANTIALLY BINDER-FREE, VAPOR DEPOSITED SILVER HALIDE MIRCROCRYSTALS SUPORTED UPON AND SUBSTANTIALLY COVERING IN SUBSTANTIALLY CONTINUOUS PHASE A SURFACE AREA OF SAID SHEET, SAID STRATUM HAVING A THICKNESS OF FROM ABOUT 0.1 TO ABOUT 0.5 MICRON AND A DENSITY OF LESS THAN THAT OF SAID HALIDE IN SOLID CRYSTALLINE FORM.
 13. A METHOD OF PHOTOGRAPHY COMPRISING EXPOSING TO AN OPTICAL IMAGE A LAYER OF SUBSTANTIALLY BINDER FREE SILVER HALIDE MICROCRYSTALS SUPPORTED ON A SUBSTRATE, SAID LAYER BEING FROM ABOUT 0.1 TO ABOUT 0.5 MICRON THICK, DEVELOPING SAID LAYER UNTIL SUBSTANTIALLY ALL OF THE LIGHT STRUCK AREAS ARE DEVELOPED THROUGH THE THICKNESS OF THE LAYER DOWN TO THE SUBSTRATE, AND FIXING THE DEVELOPED IMAGE. 